In accordance with the bond resolution, the county trustee shall place in a special account, known as the county highway bond account, the amounts of state-aid road funds and state aid funds received for county highway purposes, but not exceeding fifty percent (50%) of the state aid funds, pledged to the payment of the principal of and interest on the bonds. The funds shall be expended for no other purpose. When an amount has been accumulated sufficient to pay the principal amount of the bonds, plus interest, the county trustee shall not place additional funds in the account.